1. Field of the Invention
The present invention relates to a method of protecting a wafer front pattern, and more particularly, to a method of performing double-sided process capable of protecting a wafer front pattern.
2. Description of the Prior Art
MEMS devices, such as micro-sensors, micro-actuators, and microphones, have more complicated mechanical structures than semiconductor devices, e.g. micro-spindle structures and diaphragm structures, and thus must be fabricated by double-sided processes. However, double-sided processes are not standard semiconductor processes, and thus the manufactures face a lot of difficulties. Specifically, double-sided processes are performed with front processes including a deposition process, lithography process, and etching process, to form a front pattern on the front surface of a wafer. After that, the wafer is turned over and back processes are performed to form a back pattern on the back surface of the wafer. Double-sided processes are for the purpose of fabricating the essential structure of the device. Before performing double-sided processes, the wafer front pattern must be protected from damage during the subsequent back processes or transportation.
Please refer to FIG. 1 through FIG. 3. FIG. 1 through FIG. 3 are schematic diagrams illustrating a conventional method of performing double-sided processes. As shown in FIG. 1, a wafer 10 including a front surface 12 and a back surface 14 is provided. The front surface 12 of the wafer 10 has undergone front processes, such as a deposition process, lithography process, and etching process, to form a front pattern 16. The front pattern 16 comprises a plurality of holes 16A and 16B with different ratios of depth to width. As shown in FIG. 2, a photoresist 18 is spun on the front surface 12 of the wafer 10 to protect the front pattern 16. As shown in FIG. 3, the wafer 10 is turned over and then attached and fixed by an electrostatic chuck (E-chuck) 20 in order to proceed with the back processes.
As shown in FIG. 2 and FIG. 3, however, the photoresist 18, which works as a mask, has a higher viscosity. While the holes 16A of the front pattern 16 have a large ratio of depth to width, the photoresist 18 cannot penetrate the holes 16A completely which results in gas (bubbles) 18A formation. Under these circumstances, the gas 18A will expand by heat and cause a “popcorn effect” during a heating process of the back processes or a process of a higher temperature. The popcorn effect can result in cracks on the surface of photoresist 18 which thus loses the ability to protect the front surface 12 of the wafer 10. This also leads to the attachment and fixation of the wafer 10 by the electrostatic chuck 20 to be more difficult. In addition, if the gas 18A is located near structure of the device, it can damage the structure.
In light of the drawbacks of the above-mentioned method, especially to a wafer having a large ratio of depth to width, the yield is required to be improved.